Generally, an electronic control unit including two computers is known in which each computer monitors whether or not the other computer is operating normally. For example, an electronic control unit described in Japanese Patent Laid-Open Publication No. Hei 10-83321 (1998) controls a throttle valve of an internal combustion engine in a vehicle and includes a main CPU and a sub-CPU, wherein the operation of the main CPU is monitored by the sub-CPU. More specifically, the same temperature of coolant, the same accelerator pedal opening degree and the same position of a transmission gear in the internal combustion engine are input in the main CPU and the sub-CPU. The main CPU and the sub-CPU calculate a degree of throttle opening using the same arithmetic expression. Then, the sub-CPU receives the main CPU throttle opening degree calculated by the main CPU and determines whether or not the received main CPU throttle opening degree is coincident with the sub-CPU throttle opening degree calculated by the sub-CPU.
Moreover, in order to determine whether or not such a monitor routine of the main CPU is performed appropriately, the sub-CPU executes a determination routine for the monitor routine logic. In this determination routine, a sub-routine for the aforementioned monitor routine is executed using dummy data of the temperature of coolant, the accelerator pedal opening degree, and the gear position in the internal combustion engine. Then, the degree of throttle opening calculated by execution of that sub-routine is compared to an expected value of throttle opening prepared in advance to correspond to the aforementioned dummy data. When the degree of throttle opening calculated based on the dummy data is coincident with the expected value prepared in advance in that comparison, the monitor routine logic is determined to function normally.
However, according to the conventional technique mentioned above, it is determined only by the sub-CPU whether or not the monitor routine logic functions normally. Therefore, in a case where a determination command (comparison command or the like) in the sub-CPU is abnormal, there is a possibility that correct determination will not be performed. Thus, the conventional technique has a problem in that reliability of monitoring the operation of the main CPU by the sub-CPU is not sufficiently ensured.